High-frequency cavity resonator apparatus



u y 1950 FEENBERG 2,514,383

HIGH-FREQUENCY CAVITY RESONATOR APPARATUS Filed Aug. 25, 1944 2 Sheets-Sheet 1 NPUT 3 PHASE SHIFTER FIG.| a

INVEN TOR. EUGENE FEENBERG ATTORNEY July 11, 1950 E. FEENBERG 2,514,383

HIGH-FREQUENCY CAVITY RESONATOR APPARATUS Filed Aug. 25, 1944 2 Sheets-Sheet 2 INVENTOR. EUGENE FEENBERG ATTORNEY Patented July 11, 1950 HIGH-FREQUENCY CAVITY RESONATOR APPARATUS Eugene Feenberg, New York, N. Y., assignor to. The Sperry Corporation, a corporation of Delaware Application August 25, 1944, Serial No. 551,210

15 Claims.

The present invention relates to the art including electron discharge devices, and especially such devices utilizing cavity resonators inductively coupled to electron streams. I

Prior art devices of this general type are exemplified by Varian Patent No. 2,242,275 for Electrical Translating System and Method, issued May 20, 1941. In devices of this type, an electron stream is acted upon by an alternating high frequency electric field which velocity modulates the stream. Thestream is velocity-grouped by subsequently traversing a field-free drift space and is thereby caused to periodically vary in amplitude at the field frequency and/or harmonics thereof. In this way, an alternating current component appears in the electron current which is of the same frequency as that of the impressed field, and may be accompanied by components of frequencies harmonically related to this frequency. This variable electron current or stream is then passed through an output or energy-extracting cavity resonator, which extracts high frequency energy from the stream at or near the resonant frequency of the extracting resonator. When this output resonator is tuned to the same frequency as that of the input field, the apparatus. serves as an amplifier to amplify the input supplying the alternating field originally controlling the stream. However, when the extracting chamber is tuned to a harmonic of the input frequency, it serves to extract harmonically related energy, and the apparatus serves as a frequency multiplier, as described more in detail in Hansen and Varian Patent No. 2,281,935 for Modulation System, granted May 5, 1942.

The present invention is concerned with amplifier or frequency multiplier devices using output cavity resonators or other tuned circuits inductively coupled to an electron stream and having materially increased efiiciency and power output.

It is especially useful in devices wherein the inwhereby greater amounts of energy and greater efficiencies can be obtained.

It is still another object of the present invention to provide improved cavity resonator apparatus for extracting high frequency energy from periodically varying electron currents by the use.

of a pair of cavity resonators coupled to the stream at closely adjacent points, these resonators being adjusted to oscillate at the same frequency but with a relative phase shift corresponding to the average transittime of electrons of the stream between said resonators.

Other objects and advantages will become apparent from the following description and drawings, wherein:

V Fig. 1 is a longitudinal cross-sectional view of one form of the present invention, the section being taken along line l-l of Fig. 2;

Fig. 2 is an end view of the apparatus of Fig. 1;

Fig.2 shows a longitudinal cross-sectional view of a modification of the portion of Fig. 1 to the left of line AA thereof; and

Fig. 4 shows a longitudinal,cross-sectional view of a modification of the portionof Fig. -1 to the right of line B-B thereof.

Referring to the drawing, wherein like reference numerals areused in the several figures to designate similar elements in the respective figures, the present invention is illustrated in the form of an electron discharge device of the velocity modulation type, although the present invention is not restricted thereto. This device includes an electron velocity-modulating cavity resonator ll having a pair of electron-permeable grids l2 and [3, at opposite walls thereof defining a narrow gap l5 through which an electron stream from a cathode arrangement M (which may be of any conventional type) may be projected under the influence of a suitable accelerating voltage connected between cathode l 4 and the usually grounded entrance or accelerating grid I 2 of resonator I I. An output or energy-extracting resonator I6 having an entrance grid IT and an exit grid I8 defining a second gap 28 is spaced from the input resonator I I by a tubular member I9 surrounding the path of the electron stream and providing a field-free drift space for the electrons of the stream after their passage through the gap I5.

A second output resonator 2I is provided having a common wall 22 with respect to resonator I6 and utilizing the same grid Ii8 .as the. entrance grid thereof, and having a further grid 23 as an exit grid, thereby defining a third gap In this way the gap 25 between grids I8 and 23 of resonator 2| is closely adjacent to the gap 20 between grids I1 and I8 of resonator I6. Desirably, in velocity modulation devices such as in Fig. 1, these two gaps 20 and 25 are located as closely together as is practical in order that the electron current flowing across both gaps may be in substantially the same bunched condition. The electron stream, after passage through gaps ,2?! .and 25 is permitted to impinge on end wall 24, and is thereby collected, the energy of its electrons being dissipated in the form of heat.

Su table high frequency energy. of the resonant fr ouencv of input resonator II or diff ring therefrom but slightly in frequency, is supplied to input resonator II by way of the input coupling coaxial ne 28 terminating within resonator II in coupling loop 21. A similar coupling line .28 and coup ing loop 29 are provided for first ou put resonator I6, and a corresponding coupling line SI and loop 32 are provided for second output resonator 2I Resonators I6 and 2| are coupled together for purposes which will be described in detail below.

Such coupling is effective partly as inherent coup ing by virtue of the closeness of the gaps 20 and 2.5 and partly by a suitable phase shifter 33 of any conventional type which may be connected between the coupling lines 28 and 3| of resonators l6 and 2]. Output energy ,is derived in Fig. 1 by a similar output coupling line 34 and coupling loop 35 coupled to resonator .2 I

Apparatus is provided in the device of the present invention for independently tuning each of the three cavity resonators I I, I6, 2 I, For this purpose resonator II is provided with a flexible wall, which may bein the form of a circularly corrugated diaphr gm 36. This wall 35 is effectively interposed between grids I2 and I3 to permit adjustment of the length of gap l5. Resonator I6 is provided with a similar flexible wall 3'! permitting relative change of separation between grids I1 and I 8 and adjustment of gap 29. Also, resonator 2I is provided with a corresponding flexible wall 38 permitting relative displacement between grids It and 23 to adjust gap 25.

A hearing plate member 39 is rigidly connected to grid 12 by virtue .of its connection to the outer wall of resonator .I I A second plate member 4| is rigidly connected .to grids I3 and I] by its connection to the drift tube member L9 carrying these grids I3 and II. A third plate member 42 is rigidly connected to grid I8 through its connection to the common Wall 22 and outer walls of resonators I6 and 2I. The collector electrode 24 is formed as part of a fourth plate member 43 fixed with respect to grid 23.

It will be understood, therefore, that a change in the spacing of plates 39 and III will correspondingly vary the length of gap I5, while variation of the spacing of members M and. 42

will vary the gap 20, and variation in the spacing of members 42 and 43 will vary the gap 25. Such gap variations in these resonators cause corresponding changes in their resonant frequencies, producing tuning thereof.

For the purpose of adjusting the gap 25 and thereby tuning resonator H, a set of tuning screws 44, shown in the present instance as three in number, are threadedly engaged in plate 43 and have rounded ends 46 bearing against bearing plate member 42. Similarly, a set of tuning screws 41 are provided threaded in plate member 42 and bearing against plate member M. A corresponding set .of tuning screws 48 are threaded in member GI and bear against plate 39. In order to permit access to the tuning screws 41 and 48, the plate members 42 and 43 are suitably apertured in alignment with the screws 41 and 48, as shown at 49 and 59, permitting insertion of a screwdriver or othertool for changing tunin In this way, by suitably adjusting the tuning screws, 47 or 48, the corresponding resonators 2|, I6 and II may be individually tuned. Tension springs 5| are coupled between members 43 and 39 to take .up anyslack in the tuning system and to providepositive tuning control at all times for both directions of tuning variation. Members M and 2 are suitably apertured or cut away as shown at 52 and 53 to permit free passage of the spring52 between members 39 and 4.3.

In operation, input resonator II is preferably tuned to the same frequency as that of the energy supplied thereto. In this manner the electron stream passing through the gap I5 between grids I2 and I3 is velocity modulated at this input frequency. Subsequent passage of the electrons of thestream through the field-free drift space provided by drift tube I9 serves to transform the velocity modulation of the stream into a periodic current variation having a current component of the input frequency. Preferably, the accelerating voltage applied between cathode I4 and entrance grid I2 is selected or adjusted in relation to the length of the drift tube l9 to provide substantially optimum bunching of the electrons of the stream as the electrons of the stream reach the gaps 2H and 25. Since these gaps are located closely adjoining one another, the bunching of the stream will remain substantially optimum during passage of the stream through both gaps.

The first output resonator I5 is also preferably tuned substantially to the input frequency and serves to extract high frequency energy from the stream as the stream passes through the gap 25 between the grids I] and I8. The amount of energy thus derived from the stream is largely dependent upon the magnitude of the alternating current component of the electron stream and upon the eifective alternating voltage across the gap 28 traversed by the stream. In a resonator such as I6, which is excited only by the stream, this voltage is dependent on the stream current and the effective shunt impedance of resonator I6. This shunt impedance is limited in value by the size of resonator I6 as determined by its frequency, and by the necessity of having the gap 29 relatively short to assure the passage of the electrons through the gap in a time interval of a smaller order of magnitude than one-half period of the input frequency. For this reason, the actual extraction of energy by a single resonator, such as resonator I6, produces an efficiency materially lower than the theoretical maximum limit of 58 per cent efficiency, especially where the alternating current component of the electron stream is of small amplitude.

Because of the high merit factor Q of the resonator IE, only a very small part of the energy it extracts from the electron stream is required to maintain resonator It in oscillation. The rest of the extracted energy passes over to resonator 2| through the inherent coupling between the two resonators IB and 2 l, and through the coupling phase shifter 33. This inherent coupling necessarily exists because grid It cannot provide complete or perfect shielding between the two resonators while still permitting passage of the electron stream therethrough.

Especially where the alternating component of the stream current is small compared to the average current, the passage of the stream through the electric field in the gap 29 between grids l1 and i8 does not materially alter the magnitude of this alternating current component, although the electron velocities are changed somewhat. Accordingly, by passing this stream through the further output gap 25 between the grids l8 and 23 and having the second output resonator 2i tuned substantially to the input frequency, further energy can be extracted from the electron stream by resonator 2 If resonator 2! were independent from resonator 16, it could extract only about the same amount of energy from the stream as resonator [6. However, resonator 2| is excited not only from the electron stream, as is resonator it, but, in addition, is excited by energy from resonator l6, through their inter-coupling. Therefore, the effective high frequency voltage across gap 25 is much higher than that across gap 26. resulting in much higher energy extraction from the stream by resonator 2!. The load coupled to resonator 2| is thus supplied with this enhanced energy extracted from the stream at gap 25, and also with energy extracted from the stream at gap 20 and transferred from resonator Hi to resonator 2 I.

As is well known, for optimum extraction of energy from an electron stream, the electric field acting on the electron stream should have its maximum retarding effect at the same instant that the electron stream has its maximum instantaneous current value. When an output resonator, such as I6, is exactly tuned to the frequency of the electron stream current and is not externally excited, this phase relationship is automatically maintained. However, if resonator 25 is also accurately tuned to the stream current frequency and is assumed to be not externally excited, the oscillations of the field in resonator 2| will be phase-shifted with respect to oscillations in resonator Iii, since a finite time interval is required for the electron bunches or current peaks to travel from gap 20 to gap 25. This time interval is an appreciable part of one cycle; for example, at a wavelength of cm. corresponding to a frequency of 3000 meg-acycles per second, an electron stream accelerated by volts will travel only 0.5 cm. during one cycle. Even with a spacing of 0.1 cm. between gaps 28 and 25, this will produce a transit angle of 72 electrical degrees, which is a substantial phase shift.

Because of this phase shift, the addition of energy to resonator 2! from resonator [6 will ordinarily not result in purely arithmetic addition, but a vector resultant field will be produced in resonator 2! which no longer has the optimum phase relation with respect to the electron current variations, and the output is seriously reduced. Forthis reason, the phase shifter 33 is used. The phase shifter 33 is adjusted or selected to phase delay the energy traveling from resonator It to resonator 2! by the same phase shift as is produced by the transit time of the electron bunches between the two output resonators, or differing therefrom by a whole number of full cycles. In this way, the energy added to resonator 2! from resonator I6 will be in phase coincidence with that extracted by resonator 2| from the stream, resulting in maximum alternating voltage across gap 25 and greatly increasing the effectiveness of resonator 2| in extracting energy from the stream.

In this way, with both output resonators tuned to the stream current frequency, the proper phase relations are maintained for both resonators to remain at optimum operation, and markedly increased outputs and efficiencies are obtained.

For effectiveness of energy transfer between resonators l6 and 2!, it is desirable that their coupling be quite tight. For this reason the inherent coupling through grid I8 is not in any way harmful, as it would be in other types of tubes, such as oscillator -buffers, but is actually desirable. Usually, however, the inherent coupling is not tight enough. Accordingly, phase shifter 33 is designed to provide tight coupling while retaining the necessary phase delay discussed above. For this reason, phase shifter 33 is provided with as little attenuation as possible. A preferred form of such phase shifter is merely an adjustable length coaxial transmission line, such as is well-known in the art.

The desired condition of optimum operation can be attainedin another manner. As is known in the art, the phase of oscillation of a tuned circuit with respect to its driving voltage can be varied (within limits) by detuning the circuit 7 shifter 33 is unnecessary and is omitted.

By either of these two modes of operation, both resonators i6 and 2! may operate at substantially optimum condition for extracting high frequency energy from the electron stream. The combined output, which may be derived from either one of two resonators, as described above, will be substantially larger than the output possible from prior art devices of the type shown in the above-mentioned Varian patent.

The present invention 'is not restricted to two output resonators. Three or more such resonators may be used, suitable means, such as described above, being provided for coupling the output resonator with proper phase delay. Thus, the first output resonator is excited by the stream, and serves to partially excite the second resonator in phase coincidence with the excitation of this second outputresonatorproduced by the stream. The second output resonator in turn partially excites the third output resonator in similar phase coincidence. Each further output resonator is excited to higher amplitude of oscillation, until the desired output level is reached. The last output resonator is then coupled to the load circuit.

The apparatus of the present invention has a special advantage when the excitation of the input resonator II is of low level. Under these conditions, the high frequency variations in the electron stream are of low amplitude relative to the average electron current. This varying electron current is then able to excite a first output resonator such as It only with low amplitude of oscillation. This low amplitude of oscillation has but little effect upon the electrons of the stream during their passage across gap 28, and accordingly the stream may then serve to excite the second output resonator 2! or further output resonators with a substantal. amplitude of oscillation. The addition of energy to resonator 2| from resonator l8 markedly increases the energy extraction by resonator 2|. In this Way, a substantial increase in output is derived. Such conditions of small variations of stream current may occur with low input levels in high frequency amplifier devices, such as shown in Figs. 1 and 2, and also occur generally in frequency multiplier devices, which inherently produce small amplitudes of high frequency current variation in the electron stream even for substantial input levels.

Such a multiplier device is illustrated in Fig. 3 which is intended to replace the portion of Fig. 1 to the left of line AA thereof. Thus, instead of an input resonator H tuned to the same frequency as the output resonator l5 and ill in Fig. 3, there is shown an input resonator H which is tuned to a sub-harmonically related frequency. Resonator H is formed by a pair of concentric tubular conductors BI and 62 fixedly joined at one end by an end plate 53. At the other end of conductor BI is the bearing plate 39' which carries a diaphragm 36' concentrically arranged therein. Plate 39' serves the same function as plate 39 of Fig. 1; that is, it is a bearing plate for the tuning screws 48 and for connection to the springs 5i. The drift tube is is sealed to and supported within the diaphragm 3B and carries the grid 13 as in Fig. 1. Also carried at the end of the drift tube I9 is a capacity loading plate 64 which cooperates with the plate 65 coupled at the end of tubular member 62. Grid l2, forming the gap I 5 with respect to grid I3, is carried at the center of plate 66. Plates 65 and 64 provide an effective lumped capacitance for the resonator H which permits its physical size to be reduced while retaining its resonance at the sub-harmonic frequency. A cylindrical capacity-loading member 61 is fixed at the outer periphery of or may be integral with plate 66, and in association with the inner wall of tubular member Bl provides further capacity loading for the resonator to permit further reduction in physical size.

The cathode structure M is suitably supported on members 53 adjacent entrance grid 12, as in Fig. 1. A similar input coaxial line terminal and coupling loop 21' are utilized for exciting the low frequency input resonator l l. Tuning of resonator H is effected in the same manner as for resonator H, by varying the gap I5, as permitted by diaphragm In operation, resonator H is excited at or near its resonant frequency and produces a corresponding velocity modulation of the electron stream passing through the gap l5. Subsequent passage of the electron stream through the drift space provided by drift tube I9 produces bunching of the electrons so that at the time of their arrival at gap 2|, the electron current will have impressed thereon periodic variations at frequencies harmonically related to the frequency input of resonator ll. The resonators l6 and 2| are tuned to a desired harmonic of this input frequency. The operation of resonators l6 and 21 is then exactly the same as described above with respect to Fig. I, and either by proper choice of phase shift between these resonators or proper choice of deturning thereof, will produce the greatly increased ouput of the present invention.

Fig. 4 shows a further modification of the invention intended to replace the portion of Fig. 1 to the right of line BB. The apparatus of Fig. 4 may also be used with the device of Fig. l modified as described with respect to Fig. 3. In the form of apparatus shown in Fig. 4, the external phase shifter 33 is no longer used. Instead an internal coupling loop H is provided for tightly coupling resonators l6 and 2|. By the proper choice of the size of the coupling loop, that is, of its encompassed area within each of the two resonators, the coupling thus produced between resonators l6 and 2! may be selected to have the desired tightness, producing optimum operation. The proper phase shift between the two output resonators may be ob tained by relative detuning, as in Fig. 1. However, since the output is here derived from resonator it by way of output concentric terminal 34, energy flows from resonator 2! to resonator it, instead of in the reverse sense as in Fig. 1. Therefore, the relative detuning must be in opposite sense, to phase advance the energy flowing from resonator 2| to resonator IE, to be in phase coincidence within resonator [6.

It is to be understood that, in Fig, 1 also (and in Fig. l modified as in Fig. 3), output energy may be derived from resonator l6 instead of 2|. In such case, phase shifter 33 must provide a phase advance of the same amount as the transit time phase delay, instead of a phase delay as in Fig. l as shown.

Accordingly, by the present invention, there is provided improved high frequency apparatus from which, especially for low level input values,

greatly increased efficiencies and output amplitudes may be derived.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. High frequency apparatus comprising means for producing an electron stream and providing a. path therefor, an input cavity resonator having an electron-permeable gap along the path of said stream for velocity modulating the electrons thereof, a drift tube surrounding said stream path beyond said input resonator for causing said modulated electrons to become velocity grouped, a first output cavity resonator having an electron-permeable gap along the path of said stream beyond said drift tube, a second output resonator having an electron-permeable gap along said stream path closely adjacent to said first output resonator gap to provide close inherent coupling therebetween, the distance between the gap of said first output resonator and the gap of said second output resonator being less than one-fourth the length of said drift tube, means further coupling said two output resonaenergy therefrom.

corresponding to the average electron transit time between the gaps of said two output resonators.

3. Apparatus as in claim 1, wherein one of said output resonators is tuned Slightly below resonance with respect to high frequency current variations in said stream and the other of said output resonators is tuned to a frequency slightly higher than said electron current frequency, to maintain a phase difference between the fields of said two output resonators.

resonator is tuned to a predetermined frequency and wherein said output resonators are tuned respectively above and below said predetermined frequency.

5. Apparatus as in claim 1, wherein said input resonator is tuned to a predetermined frequency and wherein said output resonators are tuned respectively slightly above and below a harmonic of said predetermined frequency.

6. High frequency apparatus comprising means for producing an electron stream and providing a path therefor, an input cavity resonator having an electron-permeable gap along the path of said stream for velocity modulatin the electrons thereof, a drift tube surrounding said stream path beyond said input resonator for causing said modulated electrons to become velocity grouped, a first output cavity resonator having an electron-permeable gap along the path of said stream beyond said drift tube, a second output resonator having an electron-permeable gap along said stream path adjacent to said first output resonator, the electron-permeable gaps of said first and second output resonators being located along said drift tube in the region in which said electrons attain substantially maximum velocity grouping, means further coupling said two output resonators together, and means coupled to one of said output resonators for extracting high frequency energy therefrom.

7. High frequency apparatus comprising means for producing an electron stream and providing a path therefor, means along the path of said stream for velocity modulating the electrons of said stream and causing said velocity modulated electrons to become density modulated at a predetermined high frequency, a pair of coupled tuned circuits positioned along said stream path in energy-exchanging relationship thereto, said tuned circuits being located along said path at the position at which said electrons reach substantially maximum density modulation, one of said tuned circuits being tuned slightly above said predetermined frequency and the other being tuned slightly below said predetermined frequency, and means coupled to one of said circuits for obtaining output energy from said apparatus.

8. Apparatus as in claim 7, wherein said coupled tuned circuits comprise a pair of cavity resonators having a common wall and a coupling 100p extending through said wall and within each of said resonators for intercoupling the same.

9, High frequency apparatus comprising i means for producing an electronstream and providing a path therefor, means along said path responsive to an input electromagnetic wave for producing periodic current variations of said stream having a component of a predetermined frequency, a pair of tuned circuits positioned along said stream path in inductive energyexchanging relationship thereto and tuned substantially to said frequency, said pair of tuned circuits being located along said path at the position at which said periodic current variations 'reach substantially maximum amplitude, and

phase-shifting means coupling said two circuits for producing a phase difference between the oscillations of said two circuits corresponding to the average electron transit time between said circuits.

10. Appartus as in claim 9, wherein said predetermined frequency is harmonically related to 2o 4. Apparatus as in claim 1, wherein said input that of said input wave.

11. The method of producing high frequency energy comprising the steps of producing an electron stream along a predetermined path, producing periodic current variations of said stream at a predetermined high frequency, exciting a. first cavity resonator by said variable-current stream at a given point of said path, excitin a second cavity resonator by both a phaseshifted version of said first extracted energy and by said variable-current stream at a point of said path closely adjacent said first resonator excitation point, and supplying energy from said two resonators to a utilization device.

12. Apparatus for producing high frequency energy comprising means for producing an electron stream flowing along a predetermined path, means for causing said electron stream to become density modulated, and means for extracting high frequency energy from said stream, said last-named means comprising a pair of closely spaced cavity resonators having a pair of adjoining electron-permeable gaps, each gap being defined by a pair of electron-permeable electrodes located along the path of said stream and one electrode of one gap being common with one electrode of the other gap, the two gaps being located at the position at which said electro stream reaches substantially maximum density modulation.

13. Apparatus for producing high frequency energy comprising means for producing an electron stream flowing along a predetermined path, means for producing a high frequency current component in said stream, three closely spaced electron-permeable electrodes mounted successively along the path of said stream to produce a pair of adjoining gaps between successive pairs of said electrodes, and a pair of resonant circuits each coupled between the center of said electrodes and one of the outer ones of said electrodes.

14. The method of producing high frequency energy comprising the steps of producing an electron stream flowing along a predetermined path, periodically velocity-modulating the electrons of said stream by a high frequency wave, producing periodic density variations in said stream from said velocity-modulated electrons, extracting energy at a harmonic of the frequency of said high frequency wave from said stream at a first location along said stream, extracting further energy at said harmonic of the frequency of said high frequency wave from said stream at a second location along said stream, said first and second locations being closely adjacent and located in 11 the region in which said electrons attainsubstantially maximum density modulation, combining said first extracted energy with said further extracted energy, and conveying said combined en- .ergy to a utilization device.

15. The method of producing high frequency energy comprising the steps of producing an electron stream flowing along a predetermined path, periodically velocity-modulating the electrons of said stream by a high frequency electromagnetic wave, producin periodic density variations in said stream from said velocity-modulated electrons, extracting energy at a harmonic of the frequency of said high frequency Wave from said. stream at a first location along said stream, extracting further energy at said harmonic of the frequency of said high frequency Wave from said stream at a second location along said stream, said first and second locations being closely adjacent and located in the region in which said 20 REFERENCES CITED The following references arefof record in the file of this patent: c

UNITED STATES PATENTS Number Name Date- 2,280,824 Hansen et al. Apr. 28, 1942 2,281,935 Hansen et al May 5, 1942 2,294,942 Varian et a1 Sept. 8, 1942 2,305,883 Litton- Dec. 22, 1942 2,311,520 Clifford Feb. 16-, 1943 2,375,223 Hansen et al May 8, 1945 2,422,685 7 McRae 1 June 24, 1947 

